Method of drying moisture-containing materials



July 27 1926.

K. REDMAN METHOD OF DRYING MOISTURE CONTAINING MATERIALS 1925 2Sheets-Sheet 1 Filed Feb. 24

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WMMEZKQ/M July 27 1926.

K. REDMAN METHODOF DRYING MOISTURE CONTAINING MATERIALS 1923 2Sheets-Sheet 2 Filed Feb. 24

W wwmg each of the cooling I Patented July 27, 1926. i

UNITED STATES 1,593,598 PATENT OFFICE.

-KENNETH BEDMAN, OF WEST ROXBUBY, KASSACHUSETTS, ASSIGNOR, BY MESNE AS-SIGNMENTS, TO B. F. STURTEVANT COMPANY, 015 BOSTON, MASSACHUSETTS, ACOR- POBATION OF MASSACHUSETTS.

METHOD OF DRYING MOISTURE-CONTAINING MATERIALS.

Application filed February 24, 1923. Serial No. 621,059.

The present invention relates to a method of drying moisture-containingmaterials and is more particularly concerned with the kiln drying oflumber. n

It has been appreciated by those skilled inthe art for a considerablelength of time that the drying of moisture-containing materials shouldbe initiated in a relatively humid atmosphere in order that the dryingaction might be caused to take place gradually, thereby obviating thedanger of crack ing or warpin of the drying material by drying the surace too rapidly. To this end. the older methods of drying lumber consistin merely'subjectin the lumber to' heated air or other gas at Tig hrelative humidity, this relative humidity being gradually decreased andthe temperature being increased as the drying action progresses. Thismethod, however, is rather slow, and according ly possesses thedisadvantage that long continued heating periods are required, therebyutilizing excessive amounts of fuel to say nothing of the chargesinvolved in the long use of the. kiln for one drying operation.

The principal object of the present invention is to provide a method ofdrying Wood or other moisture-containing materials whereby the totaldrying operation may be much more rapidly performed with an increasedqualit of product.

Another ob ect of the-invention is to provide a method of dryingmoisture-containing material by which the entire drying operation may bemore economically effected than in revious existing methods.

With t ese and other objects in view, the present invention consists ofthe method or steps hereinafter described and more particularly definedin the claims.

I have discovered that the drying of lumher or similar material may begreatly improved by alternately heating and cooling the lumber andproviding after certain or erlods an extended rest period during whichthe lumber is not subjected to currents of either hot or cold air.Accordingly, the principal feature, of the present invention is theprovision of the following cycle of operations in the drying of lumber:First, subjecting the lumber to a current of heated air or other gas ofhigh humidity; second, passin a current of relatively cool air over theumber; and third,

allowing the lumber to remain in a still atmosphere for a considerablelength of time. The cycle of operations may then be repeated, it beingpreferable, however, to lower the relative humidity and increase the drybulb temperature in the first step of the cycle.

It is believed that the advantages accruing to this method dependlargely upon differences in vapor tension which'may exist betweenmoisture in the heated material and that of the surrounding air or gasand also to certain hygroscopic efl'ects existing within the materia Forexample, the initial step of assing heated air of high relative humi ityover the lumber thoroughly heats the lumber to a fairly high temperatureand allows only partial drying of the wood, at the same time preventingexcessive evaporation of the moisture from the-surface. The vaportension ofthe moisture at the surface or shell of the wood is thus maderelatively high. The second step which consists in passing relativelycool air over the lumber allows evaporation of the moisture from thesurface of the lumber into the current of air, due to the lower vaportension of the moisture in the air. The draft of air may then be shut OEand the lumber subjected to a rest period for a considerable length oftime durin which a transfusion of the moisture ta es place from thecentral portion of the material to the outer surface, due to the factthat the central portion of the wood still retains its higher moisturecontent which tends to equalize with theisurface of the lumber which haspreviousl had its moisture content materially reduce during thepreceding step. More moisture may then be removed from the wood byrepeating the above cycle of. operations as many times as may bedesired. It has been found that this method of inserting a rest periodinto the cycle of operations not only1 reduces the operating timerequired for t e kiln drying but produces an improved quality of productwith greatly less liability of checking or otherwise injurin the lumberdue to improper kiln operatlon. The method also ossesses the advantageof re uiring materially less heat and power for e entire o erationi ,h b

Be ore app ying any 0 .t e a ove steps, it is highly desirable, althoughnot necessary, that the material to be dried be heated throu bout to afairly hi h temperature but w1t-hout an substantial rying. To this end,another fzature of the invention provides for subjecting the material toa pre-.

I at the start of the drying operation.

Certain other features of the invention consist of certain novelfeatures of construction and modes of operation hereinafter describedand claimed, the advantages of which will be apparent to those skilledin the art from the 'fo'llowlng description.

In the accompanying drawings Figure 1 is a plan view in section of a fantype or forced draft kiln in which the im roved' method of the presentinvention may I ried out; Fig. 2 is a section on line 2-2 of Fig. 3showin endwise piling of the lumber in the kiln; of the kiln shown inFig. 1; and Fi 4 is a sectional elevation of a common orm of pipe coilkiln showing crosswise piling of the lumber. I y

The present invention resides in the cycle of operations andcombinations of steps'in the method of dryin and the particular type ofapparatus use may be of the usual formowith certain slight modificationswhich will hereinafter appear. The illustrated apparatus for carryingout the improvedmethod comprises both forced draft kilns and pi e coilkilns with either of which the metho of the present invention may beempltliyed.

eferring to Figs 1, 2 and 3, the forced draft kiln of the usual formconsists of side walls 6, end walls 8, roof 10, floor 12 and suitabledoors 14. The lumber is referably piled on trucks 16 and 18, the dierent layers of lumber being separated in the usual manner by stickers20. In order that the air or gas may penetrate to all parts of the wood,the pile 1s preferably constructed with a central ta ring flue 22, asindicated in Fig. 2. e trucks 16 and 18 are run into the kiln over rails24. The circulation of air or other gas in" the kiln is effected by afan or blower 26 havin an inlet connected with heating coils 28 anahumidity s rayer 30, and an outlet connected with supp y assages 32 and34. The assages 32 an 34 communicate with supp y ducts 36 and 38,-

respectively, which are tapering in form and are located beneath thetrucks. The ducts are rovided with louvers 40 and 42 of the usu formconsistin of a series of u standing metallic vanes. t will be seen t atthe carig. 3 is a sectional elevation through the assages 32 and 34upwardly through the ues 22 by the louvers. A deflector 44 may beinserted at the joint between the passages 32 and 34 to control thedownwardly through openings 46 in the floor 12 into return ducts 48which are connected at openings 49-, and thence into an intake duct 50leading directly to the inlet of the heater 28. In order to allowcirculation of fresh air through the kiln, ventilators 52 and 54 areprovided, the former leading directly to the inlet of the heater, asindicated Hi Fi ."1, and the latter 'being meraly connecte with anopening in the roof 10. The ventilator 52 is provided with a slide 56and the ventilator 54 is provided with a damper 58 which may be actuatedin any desired manner. Upon opening the slide 56 and damper 58 thecirculation of fresh air from the atmosphere may be caused to take placewithin the kiln.

The humidity sprayer is fed with steam through'a steam p1pe 60 and asuitable valve valve 64 to perforated steam pi es 66 and 68 resting uponthe floor 12 for t e purpose of injectlng live steam into the kiln forthe preliminary step of thoroughly heating the lumber. The completeseries of operations for d 1n lumber in the above desired type of kil nwi now be enumerated in detail. The prel mmary step consists of theadmission of hve'steam to thoroughtly heat the lumber 62. The steam p1pe60 also leads through a through its mass. The average time for thisoperation is from two to four hours for one lnch lumber and will, ofcourse vary with the thickness of the lumber and its previous degree ofmoisture. The first step in the cycle of operations consists in theadmission of humid heated air into the kiln. Tothis end,'the fan 26 isstarted and the valve 62 is o ened to permit steam to be injected into te current of air through the humidity sprayer 30, the valve 64 beingclosed. In this step, it is desirable to approximate the humidity andtemperature conditions recommended by the Forest Products Laboratory atMadison, Wisconsimof the U. S. Governmcnt Department of Agriculture. Therec- 'ommendations cover a considerable range of moisture content,thickness and kinds of lumber, and the humidity and temperatureconditions may be readily adjusted to conform therewith. During thisperiod it is air passing through the fan is directed desirable tore-circulate most of the air and remainder of the working da The secondstep of the eye e is comparatively short and consists in the rapid dryinof water vapor from the surface or shell the wood. To this end thehumidit sprayer and heater coils are turned off an slide 56 and damper58 are open wide to allow the circulation of fresh cool air through thekiln, continuing the operation of the fan 26 all the while. This stepconveniently lasts from one to two hours. During this step the vaportension of the moisture in the heated wood is much greater than that ofthe surrounding cooler air or gas. Since all pressures tend to equalize,it is believed that the vapor in the heated wood tends to create anequilibrium with the vapor tension in the cooler air. In so doin thesurface moisture of the wood is rapidly given up, thereby cooling thewood until an equilibrium is reached, whereupon all moisturetransference to the atmosphere substantially ceases.

The third and final step in the cycle of operations consists of a restperiod during which time the blower 26 is shut off. Communication withthe atmosphere is maintained by keeping the ventilators open but noforced circulation of air is necessary.

During this period, it is believed that due to the hygroscopic characterof the wood, the moisture contained in the central portions tends toestablish an equilibrium with the surface or shell which has just beendried durin descri d. That is to say, the surface or shell absorbsmoisture from the center so that at the conclusionof the rest period,the surface again contains moisture which may be readily evaporated byrepetition of the preceding cycle. This rest period preferab y continuesover a number of hours and accordingly may last all night. It will beseen that the use of heat and power during the night is thus obviatedand furthermore,

no attention is required.

Upon arrival of the workman in the mornin the kiln is once againststarted preferab y omitting, however, the preliminary steamingoperation. The heating step wit humid air is carried throu h this time,however, with a reduced relative humidity and an increased dry bulbtemperature. This is followed, as before with the drying by means ofcool air delivered by the fan, and this in turn by the rest eriod. Thecycle of these three steps may e repeated as man times as desired, thetemperature prefera ly bein increased, and the humidity decreased witheach recurrence of the first step.

It is believed that the foregoing remarks the second step of the cycleabove correctly explain the theoretical operationof the improved method.Re ardless of the correctness of this -theory, known that this methodproduces a higher quality of product than previously existing methodsand may be much more economically carried out.

A slight modification of the above rocedure may be carried out with thinlum er or veneer. In this case the passage of the moisture from theinternal to the external layers does not require considerable lengths oftime so that in the first cycle of operations the extended rest periodmay be considerably reduced or, in fact, omitted altogether. During thesecond cycle, however, the rest period should be used for maximum economand rapidity of drying.

he features of the present invention are not alone applicable to theforced draft type of kiln but may be embodied in a pipe coil kiln of theusual form. In this ty e of owever, it is kiln, the heating is done byconvectiona currentsof air which are heated over a series of pipescontaining steam or other heating fluid. Referring particularly to Fi 4,the pipe kiln is constructed with side we s 6/and roof 10, as before.The trucks 16 are preferably run in over three rails 24 if theillustrated form of crosswisepiling is employed. As before, thesuccessivelayers of lumber are separated by stickers 20. Fresh air isadmitted to the kiln throu h air ducts 70 and is allowed to pass over te rows of heated pipes 72. The air is raised to the ropcr umidity bysteam from the per orated eration is eii'ected by closing the air ducts,

7 0 and the dampers 74 and turning on the steam in pipes 66 and 68 for aperiod of two to four hours. The first heating step in the cycle is thencarried out as in the case of the forced draft kiln. The pipes 72 areheated by the passage of steam therethrough and the ducts 70 and dampers74 are opened to allow the circulation of heated air upwardl through thelumber pile, the proper humid ity being maintains by regulation of thesteam in pipes 66 and 68. As in the previous case, this operation iscarried out for the remainder of the working day. The temperature andhumidity maybe substantially the same as in the other case but it isdesirable to have the temperature slightly higher and the humiditysomewhat lower to compensate for the slower circulation. The second stepin the cycle consists, as before, in passing the cool dry air over thelumber. This is caused to take place b natural instead of forcedcirculation. T e previously heated condition of the lumber and kilnbuilding will tend to permit circulation of the air therethrough. Thesteam in pipes 66 and 68 is turned off during this step. As in theprevious case, the third and final step consists in a rest period duringwhich time a transfusion of the moisture occurs from the centralportions of the wood to the surface. In the case of a pipe coil kiln,this rest period obtains as soon as the vapor tension of the moisture inthe wood substantially equalizes with the vapor tension ofthesurrounding air or gas, that is, when the temperatures of the lumberand gas have become practically equalized. During this rest period thereis substantially no circulation of air through the kiln. It is to bepointed out that as in the case of the forced draft kiln, the severalsteps are separate and distinct and although the line of demarcationbetween the second and third steps may not be as pronounced in the pipecoil kiln as in the forced draft kiln, there is nevertheless nodifierencein the principle ofoperation between the two types of kilns asin both types the second step consists in the evaporation of moisturefrom the surface of the wood and the third step consists in thetransfusion of the moisture from the interior to the shell of the wood.The only difference is that in the case of the pipe coil kiln, thechange from the second to the third step isgradual and takes placenaturally due to the equalization of the temperatures rather thanthrough any actual o eration of the apparatus as in the case o theforced draft kiln. After the completion of the rest period, the cycle ofoperations is carried on as before, preferably omitting the preliminarvsteaming operation.

Although the resent invention has been s ecifically described inconnection with the rying of lumber, it is understood that the inventionshould not be so limited, but

should extend to the dryin of other mate rials. Theinvention shoul alsonot be considered limited to the specific steps described herein exceptas limited in scope by the appended claims.

The invention 'having been described, what is claimed is a y .1. Themethod of drying moisture-containing materials which consists insub'ecting the material to a. current of heate air or gas at highrelative humidity, then to a' current of materially cooler as, allowingthe material to-rest fora perio sufiicient to permit diffusion of themoisture from the central portion to the surface of the mateperiods torial, and thereafter subjecting the material to a current of heated air.

2. The method of drying materials which in vapor tension between themoisture at the surface of the material and the moisture in the air;third, allowing the material to rest without further drying action for aperiod sufficient to ermit transfusion of the moisture outwardly fromthe central portion of the material; and fourth, passing a current ofair over the material to absorb moisture from the surface thereof.

3. The method of drying materials which consists in repeatedly andalternately passing heated air of high relative humldity and materiallycooler air over the material and allowing the material to rest withoutfurther drying after certain of the cooling ermit transfusion ofmoisture outwardly 50m the central portions of the material.

4. A method of drying materialswhich consists in thoroughly heating thematerial throughout its mass without drying and thereafter subjecting itto the following repeated cycle of operations; first, subjecting thematerial to a circulation of humid heated air, then passing a current'ofrelatively dry air at a lower temperature over the material to permitdifferences in vapor tension to cause evaporation of moisture from thesurface of the lumber, and thereafter allowing the lumber to restwithout further drying action for an extended period to permit Itransfusion of moisture outwardly from the central portions ofthe-material.

5. The method of kiln drying lumber which consists in thoroughly heatingthe lumber throughout its mass by contact with steam, then repeatedlypassing alternate currents of heated humid air and relatively dry air ata' lower temperature over the lumber, and thereafter allowing the lumberto rest without d ing action at the conclusion of certain o the coolingperiods for extended periods to permit transfusion of moisture from thecentral portion toward the surface of the lumber.

6. The method of drying moisture-eontaining materials which consists insubjecting the material to a current of heated air or gas at highrelative humidity, then to a current of materially cooler gas until theer gas to permit the diffusion of moisture from the central portion tothe surface of memes the material, and then again subjecting thematerial to a current of heated air or gas and thereafter to a currentof cooler gas.

7 The method of drying materials which consists in heating the materialand drying, then cooling the same by subjecting it to a current of coolair, then permitting the cooled material to stand without drying, andthereafter again heating and drying the ma- 10 terial.

8. The method of drying moisture-containing materials which consists insuhjecting the material to a current of heated air or gas at highrelative humidity, then to a current of materially cooler gas,thereafter allowing the material to remain in the cooler gas and thenagain subjecting the material to a current of heated air or gas but ofhigher temperature and lower humidity.

than the former current of heated air.

KENNETH REDMAN.

